Extrusion process and apparatus for forming tubular film



Nov. 14, 1961 M. GOLDMAN 3,008,185

EXTRUSION PROCESS AND APPARATUS FOR FORMING TUBULAR FILM Filed Jan. 24,1958 2 Sheets-Sheet 1 INVENTOR MAX GOLDMAN.

ATTORNEY Nov. 14, 1961 M. GOLDMAN 3,008,185

EXTRUSION PROCESS AND APPARATUS FOR FORMING TUBULAR FILM Filed Jan. 24,1958 2 Sheets-Sheet 2 a GAS IN VENTOR MAX GOLDMAN BY fwd lb.

ATTORNEY Stats This invention relates to thermoplastic tubing and moreparticularly to an improved process and apparatus for extrudingcontinuous tubing of organic thermoplastic materials. It has beenproposed to rapidly quench freshly extruded tubing by passing the tubingdownwardly into a quench bath of coolant, e.g., water. The tubing, inthe distance between the extruder and the approximate surface of thequench bath, is maintained in the inflated state by a bubble of air, andin the bath a head of liquid is maintained in the tubing to the pointwhere the quenched tubing is converged (flattened) between pinch rolls.While this quenching procedure does, for the most part, overcome thedisadvantages of air cooling noted above, it is not altogethersuccessful for the reason that is difiicult, particularly at highextrusion rates, to maintain a stable bubble of air in the tubing, andbubble instability leads to poor convergence of the tubing andconsequent wrinkling at the pinch rolls. Moreover, surface bath rippleand sub-surface bath movement incident to circulation of the quench bath(to maintain a constant temperature) tends to oscillate and hence placestrains on the still-plastic tubing, and as a result quenched tubinghaving non-uniform wall thickness is obtained.

In the conventional manufacture of thin-walled tubing from organicthermoplastic materials by means of circular die extrusion, the tubingis extruded either vertically or horizontally. The extruded tubing isquenched while the tube is expanded in the transverse direction anddrawn from the die in the longitudinal or machine direction. Quenchingof the extruded tubing usually takes place by means of an airdistributor located externally to the tubing surface. However, quenchingby this method is slow, and as a result, circular die extrusion musttake place at low temperatures and low extruder throughputs.Furthermore, in the case of many crystallizable thermoplastic materials,this slow rate of quenching results in a film having a substantialdegree of crystallization and unbalanced physical properties, which is adistinct disadvantage when it is desired to later orient the film bystretching and/or rolling the film in two directions to substantiallythe same degree whereby to obtain a film having physical propertiesenhanced equally in both directions.

It is an object of this invention, therefore, to provide a satisfactoryprocess for the rapid quenching of thinwalled tubing of organicthermoplastic material being melt-extruded from a circular die. Anotherobject is to rapidly quench freshly melt-extruded thin-walled tubing ofcrystallizable organic thermoplastic material whereby the quenchedtubing has substantially uniform dimensions and balanced physicalproperties in both the transverse and longitudinal (machine) directionsof the tubing. Still another object is to provide improved apparatus forpro ducing thin-walled tubing of organic thermoplastic material. Theforegoing and additional objects will more clearly appear hereinafter.

These objects are realized by the present invention which, brieflystated, comprises continuously extruding molten organic thermoplasticmaterial substantially vertically downwardly in the form of tubing ofcircular cross section, continuously advancing said tubing substantiallyvertically downwardly through a zone of limiting crosssection, expandingsaid tubing in said zone to the limiting cross-section by the pressureof gas on the interior of the tubing while maintaining the temperatureof the tubing above its stick temperature, and continuously advancingsaid tubing at a temperature above its stick temperature substantiallyvertically downwardly into a bath of liquid coolant maintained at atemperature effective to rapidly cool (i.e., quench) said tubing andmaintaining a head of liquid in said tubing below the zone of limitingcrosssection, the level of said head of liquid being adjusted tomaintain the gas at a pressure effective to maintain the tubing in saidzone of limiting cross-section at said limiting cross-section.

Although the process of the present invention is applicable to allorganic thermoplastic materials capable of being melt-extruded,particular emphasis will be placed, in the description and examples tofollow, on the treatment of those polymers which are readily obtainablein crystalline form. As examples of organic thermoplastic materialswhich are extrudable in accordance with the process of this inventionthere may be mentioned: polyethylene including linear polyethylene,copolymers of polyethylene, polyvinyl chloride, polyvinylidene chloride,polyvinylidene fluoride, polyoxymethylene resin, polystyrene,polyethylene terephthalate, copolymers of tetrafluoroethylene andperfiuoropropene, as well as those crystallizable organic polymericsubstances normally not capable of being melt-extruded to whichsuflicient quantities of appropriate solvent have been added to renderthem melt-extrudable.

The term, stick temperature, is the minimum temperature at which asample of the polymer leaves a wet molten trail as it is stroked withmoderate pressure across the smooth surface of a heated block of brass.In the case of polymers capable of being readily crystallized, the termcrystalline melting point is analogous to the stick temperature and assuch will be utilized in the description and examples to follow.

This invention will be further described with reference to theaccompanying drawings wherein:

FIGURE 1 is a schematic sketch in the longitudinal section of apreferred arrangement of apparatus embodying the features of thisinvention, and

FIGURE 2 is an enlarged cross-sectional view of a portion of theapparatus illustrated in FIGURE 1.

Referring to FIGURE 1, illustrative of a preferred embodiment of thisinvention, molten thermoplastic material is extruded from a conventionalmelt-extrusion device, generally indicated by the legend A, through anannular die orifice 1 in the form of continuous tubing 2 which isexpanded by gas (e.g., air) supplied through inlet 3, and is drawnvertically down from the die and through a zone of limitingcross-section defined generally by an open-ended porous-walled cylinder4, preferably of porous metal, which is vertically (coaxially) alignedwith the extrusion orifice and which terminates at its lower end a shortdistance above the surface of quench bath 5 of water or other suitableliquid coolant, into which the tubing is continuously drawn from thezone of limiting cross-section. The quenched tubing is converged betweennip rolls 6 which are also vertically aligned in the bath with theannular die. The converged tubing is then guided from the bath overidler roll 7, and is passed over and between squeeze rolls 8 to removeexcess bath liquid, after which it is suitably dried, slit and/orotherwise processed and collected on means not shown. An overflow weir16 is provided to facilitate non-turbulent circulation of fresh waterintroduced into the bath 5 through inlets 17.

The critical feature of this invention resides in the maintenance of astable body or bubble of gas within Patented Nov. 14., 1961 the freshlyextruded tubing while the tubing is still in the plastic state, i.e., ata temperature above its stick temperature. Referring to FIGURE 2, thisis accomplished by (1) provision of a zone of limiting cross-section,which in the. preferred embodiment'comp'rises an external (with respectto the tubing) open-ended porous metal cylinder 4 mounted concentricallybelow the annular die orifice and terminating from to 2" above theliquid level of the quench bath; and (2) by an adjustable head of liquid9 maintained in the tubing at a level substantially at. the liquid levelof bath 5. Liquid is introduced into the interior of the'inflated tubingthrough an inlet line 10 and its level is adjusted by means of a liquidlevel probe 11 positioned in the extruded tubing which exerts itsaction, responsive to changes in the level of liquid in the tubing, on asolenoid valve 12 in the liquid inlet line. Compressed air is introducedin the porous cylinder through an air inlet '13. The air thus introducedis forced through the pores of the rings so as to form a thin air layer14 between the ring surface and the downwardly extruded tubing. Thisthin air barrier keeps the tubing out of contact with the porous ringsurface and serves to form and stabilize the tubing to its ultimatedesired form. By the proper adjustment of the water level inside thetubing, the bubble pressure is maintained so as to just force the tubingagainst the air barrier 14 and to permit proper convergence at the niprolls 6. In essence, this step of passing the film through the externalporous ring while maintaining it out of contact with the ring surface bymeans of a thin air barrier serves the sole purpose of forming andstabilizing the tubing to its desired' dimensions. It does not serve tocool the tubing to any appreciable extent. The temperature of the tubingupon entering the quench bath is substantially still above the sticktemperature of the polymer and thus the desired rapid quenching actionis effected on the tubing. In the case where. the polymer is extruded ata particularly high temperature or is extruded at an extremely rapidrate, it may be necessary to supply coolant through the external porousring structure in order to prevent bubble formation in the extrudedtubing or the overheating of the external porous ring. This isaccomplished by providing a tubular heat-exchanger 18 in thermal contactwith the walls of cylinder 4 through which heat-exchange fluid may becirculated.

The porous-walled cylinder is preferably fabricated of a sintcred metal,e.g., sintered bronze or the like. Instead of sintered metal, other gaspervious structures may be employed, such as for example aggregates ofsmall metal balls point-welded or soldered together so as to providethrough-going air passages relatively uniform in both size anddistribution. Or sintered wire matrices in avariety of forms, as well aspervious metal-ceramic composites characterized by thermalconductivities approaching those of metals may be employed. In a typicalapparatus, the porous-walled cylinder was fabricated from /2" thicksintered bronze metal having a porosity of about 27% on the volumetricbasis which permitted thepassage of about cubic feet per. minute persquare foot of heat-exchanger area of air at 5 pounds per square inchgauge differential across the porous wall.

The following specific examples will serve to further illustrate theprinciples, practice and advantages of the invention.

Example 1 Polyethylene having an annealed density of 0.92 gram per cc.and a crystalline melting point of 112 C. was extruded at a temperatureof 200 C. through a 1" extruder (National Rubber Machinery Co.) having abarrel opening (L/D) of 18, equipped with a Robbins 2" die, said diebeing maintained at a temperature of 200 C. and in a manner describedhereinbefore, downwardly extruded into a quench bath, the temperature ofwhich is maintained at 29 C. The film was drawn through the nip rolls ata speed of lO'feet per minute. The air pressure to the external porousring was maintained at between 3-5 pounds per square inch gauge. Theoutside diameter of the extruded tubing was 4". The diameter of theexternal porous ring was 4" inside diameter and 4 /2" outside diameter.The external porous ring was located at a distance of /z" above thewater level. The film tubing thus formed was essentially wrinkle-freeand exhibited substantially balanced physical properties.

Example 2 In the apparatus described in Example 1, both polyethylenehaving an annealed density of 0.92 gram per cc. and a crystallinemelting point of 112 C., and polyethylene having an annealed density of0.95 gram per cc. and a crystalline melting point of 133 C. weredownwardly extruded and rapidly quenched in the cooling bath. In thecase of the first-named"polyethylene, the tubing was extruded at atemperature of 200 C. The air pressure to the external porous ring was 4pounds per square inch gauge; the film Was extruded at a rate of 126grams per minute. In the case of the secondnamed polyethylene, thetubing was extruded at a temperature of 250260 C. and was drawndownwardly into the nip rolls at a speed of 7.5 feet per minute. The airpressure to the external porous ring was 5 pounds per square inch gauge.The film was extruded at a rate of 53 grams per minute. Againsatisfactorily quenched wrinkle-free tubing-possessing substantiallybalanced physical properties was obtained.

Example 3 20 C. The film was drawn through the nip rolls at a speed of13.5 feet per minute. The air pressure to the external porous ring wasmaintained at between 3-5 pounds per square inch gauge. The outsidediameter of the extruded tubing was 4". The diameter of the externalporous ring was 4" inside diameter and 4% outside diameter. The externalporous ring was located at a distance of /2" above the water level. Thefilm tubing thus formed was essentially wrinkle-free and exhibitedsubstantially balanced physical properties.

The process of the present invention, featuring the essential and novelstep of bringing the extruded and inflated tubing into con-tact with anair barrier emitted through an external porous ring structure, allowsfor the rapid quenching of an essentially wrinkle-free thermoplasticfilm tubing exhibiting a substantial degree of balance in physicalproperties and excellent clarity. It now makes feasible the preparationon a commercial scale of tubing prepared from such polymers aspolyethylene terephthalate which have heretofore been able to besuccessfully extruded only in the form of a flat sheet. The essentialadvantages of being able to prepare such a film in the form of tubingare as follows:

(1) Better balanced physical properties (in the extrusion of flat sheetsthe step of drawing the extruded film downwardly through air to a quenchdrum usually results in the film being drawn more in one direction thanthe other thus causing an imbalance of physical properties).

(2) The tubing may be oriented continuously after quenching over acircular die stretching apparatus, thus eliminating the need for costlytenter frames.

(3) There exist many direct uses for film in the form of tubing,particularly in the packaging field.

(4) Production of film in tubular form reduces waste.

An additional advantage of considerable commercial importance is that arange of tubing diameters may be produced using a single extrusion diesimply by using in each case a porous-walled cylinder corresponding tothe diameter of the tubing desired.

I claim:

1. A method for the manufacture of continuous thermoplastic thin-walledtubing which comprises continuously extruding molten organicthermoplastic material downwardly in the form of tubing of circularcrosssection, continuously advancing said tubing downwardly through azone of limiting cross-section, expanding said tubing in said zone tothe limiting cross-section by the pressure of gas on the interior of thetubing while maintaining the tubing above its stick temperature,continuously advancing said tubing without further expansion and at atemperature above its stick temperature downwardly into a bath of liquidcoolant maintained at a temperature efiective to rapidly quench saidtubing, continuously converging the quenched tubing in said bath to forma flattened tubing, and maintaining a head of liquid in said tubingbelow the zone of limiting cross-section, the level of said head ofliquid being adjusted to maintain the gas in said tubing at a pressureefiective to maintain the tubing in said zone of limiting cross-sectionat said limiting cross-section.

2. A method for the manufacture of continuous thermoplastic thin-walledtubing which comprises continuously extruding molten organicthermoplastic material downwardly in the form of tubing of circularcross-section, continuously advancing said tubing downwardly through agas pervious cylinder, expanding said tubing in said cylinder to apredetermined ultimate diameter by the pressure of gas on the interiorof the tubing, maintaining the outer surface of the expanded tubing outof contact with said cylinder by means of gas under pressure suppliedsubstantially uniformly between said outer surface and said cylinder,maintaining the tubing as it passes through said cylinder at atemperature above the stick temperature thereof, continuously passingsaid tubing at a temperature above its stick temperature from saidcylinder downwardly into a bath of liquid coolant maintained at atemperature eflective to rapidly quench said tubing, continuouslyconverging the quenched tubing in the bath to form a flattened tubing,and maintaining a head of liquid in said tubing below said cylinder, the

level of said head of liquid being adjusted to maintain the gas in saidtubing at a pressure effective to maintain the tubing at saidpredetermined diameter as it passes through said cylinder and into thebath.

3. The process of claim 2 wherein the thermoplastic organic material ispolyethylene.

4. The process of claim 3 wherein the polyethylene is linearpolyethylene.

5. Apparatus for the extrusion of thin-walled tubing of thermoplasticorganic material comprising in combination, an extrusion die having anannular extrusion orifice, an open-ended cylinder having a gas perviouswall and disposed below and in coaxial alignment with said annularextrusion orifice, a bath of liquid coolant disposed below and in closeproximity to said cylinder, means for converging tubing in said bath,means for continuously drawing tubing extruded from said orificedownwardly through said cylinder and said bath and said means forconverging the tubing, means for supplying gas under pressure to theinterior of said tubing as it is extruded, means for maintaining a headof liquid in said tubing, means for adjusting the level of said liquidresponsive to changes in the pressure of said gas in said tubing wherebyto maintain the pressure of said gas in said tubing substantiallyconstant, and means for uniformly supplying gas under pressure throughthe gaspervious wall of the cylinder.

References Cited in the file of this patent UNITED STATES PATENTS2,047,554 Fischer July 14, 1936 2,387,886 Devol Oct. 30, 1945 2,433,937Tornberg Jan. 6, 1948 2,450,457 Grotenhius Oct. 5, 1948 2,461,975 FullerFeb. 15, 1949 2,519,375 Jargstorfi et al. Aug. 22, 1950 2,541,064 IronsFeb. 13, 1951 2,632,205 Fitz Harris Mar. 22, 1953 2,641,022 Kress June9, 1953 2,698,463 Conwell et al. Jan. 4, 1955 2,814,071 Allen et al Nov.26, 1957 FOREIGN PATENTS 714,915 Great Britain Sept. 8, 1954

1. A METHOD FOR THE MANUFACTURE OF CONTINUOUS THERMOPLASTIC THIN-WALLEDTUBING WHICH COMPRISES CONTINUOUSLY EXTRUDING MOLTEN ORGANICTHERMOPLASTIC MATERIAL DOWNWARDLY IN THE FORM OF TUBING OF CIRCULARCROSSSECTION, CONTINUOUSLY ADVANCING SAID TUBING DOWNWARDLY THROUGH AZONE OF LIMITING CROSS-SECTION, EXPANDING SAID TUBING IN SAID ZONE TOTHE LIMITING CROSS-SECTION BY THE PRESSURE OF GAS ON THE INTERIOR OF THETUBING WHILE MAINTAINING THE TUBING ABOVE ITS STICK TEMPERATURE,CONTINUOUSLY ADVANCING SAID TUBING WITHOUT FURTHER EXPANSION AND AT ATEMPERATURE ABOVE ITS STICK TEMPERATURE DOWNWARDLY INTO A BATH OF LIQUIDCOOLANT MAINTAINED AT A TEM-